A typical standard networking system implemented single-function, fixed functionality. The first generation of virtualized systems offers per-customer functionality, but the functionality is still fixed. These limitations lead to several drawbacks. Customers may judge service providers based on service availability. Customers may perceive any downtime as a problem with the service provider and may consider switching providers. Service providers want to add service products to their offerings to generate more revenue and increase margins with higher-value offerings. Some of today's systems require downtime associated with upgrades. This is the case because their systems package all functionality into a single runtime image. It is simpler to design and test a system when all functionality is packaged and released in a single unit. In some cases, the service provider has to minimize downtime by building a redundant topology and taking down one system while the backup system handles service. This is non-optimal because it forces the service provider to purchase redundant hardware and design complex configurations. To achieve economies of scale and lower capital expenditures, service providers are installing systems that service multiple customers on a single system. Rather than dedicating hardware to each individual customer, the service provider amortizes that capital expense across many customers, lowering the average cost. These service providers typically schedule downtime with their customers for routine maintenance. This scheduling is more difficult when multiple customers are configured to utilize the same system.
In addition, typical networking systems may offer fixed functionality that is composed in a fixed manner. For instance, processing is usually data link layer L2 followed by network layer L3, or secure socket layer (SSL) acceleration followed by load balancing. Typically, networking systems implement fixed functionality with a monolithic version of software. Those systems that offer Virtual loading typically use a simple link-time configuration strategy or simple Virtual loading at start time, but not thereafter. Thus, you may get to choose what functionality you want to run at startup time, but you cannot change it thereafter. Typically, prior systems have had disadvantages such as they require a reboot when they are upgraded. This causes downtime. As a result, some conventional systems lack the ability to configure functionality in an arbitrary manner using an arbitrary topology, to add new functionality to a running system without causing downtime, or to upgrade a portion of functionality to a new revision.
Furthermore, in data centers, from time to time they have to upgrade the software running on a packet processing system or change the network topology or upgrade the hardware. Any upgrade of software or hardware disrupts the service availability. Such upgrades are normally done during the maintenance hours in which the service is not available to the customers. Alternatively, the service network is duplicated so that the service is available on one of networks always. In the first approach, service is disrupted. In the second approach, redundant network must be setup and maintained carefully.
Therefore, a better solution is highly desirable to be able to compose, manage, change, or upgrade a topology of a network system.